Peirce Crater: Apollo 17 vs Lunar Orbiter 4

Some of the interesting features of Peirce crater as seen by Lunar Orbiter 4 have been described previously. Lunar Orbiter 4 was at an altitude of 5503km when LO4-191H was taken, so the resolution was very low; the size of the smallest resolvable object was 117 meters at a contrast ration of 3:1. During the Apollo 17 mission, the mapping camera made two series of photographs that show the crater Peirce at much higher resolutions. These photographs obviously are important for better interpreting the topography of the crater.

Reduced-sized images from LO4-191H3 and AS17-288 are shown in Figures 1A and 1B below for comparison. It's obvious that the Apollo photograph is the more detailed of the two. The resolution at a 1000:1 contrast ratio would be 14.5 meters at the 222KM altitude of the Apollo command module when AS17-288 was taken. The crater does not appear elliptical in the Apollo image because the camera's line of sight was almost vertical to the lunar surface, while the Lunar Orbiter image was made at an oblique line of sight.approximately 15 degrees from the vertical. The support data for AS17-288 is here.

Figure 1

Click here to download A17288A.jpg and here for the image support data.

Unfortunately, the Apollo photograph was made when the sun was rising while the Lunar Orbiter photograph was made when the sun was setting, so some of the more interesting features in the Lunar Orbiter frame are in partial shadow in the higher-resolution Apollo frame, especially the diamond-shaped light region in LO4-191H3 at a position to the left of the letter "B" on both images in Figure 1. Also the sun is significantly higher in the Apollo photograph, so the length of an object's shadow is only 1/2 the length it would be in the Lunar Orbiter 4 frame.

The shorter shadows in the Apollo photograph diminishes the contrast and makes small features harder to see than they would be at lower sun angles. A higher sun angle effectively lowers the camera resolution. In analog photography, camera resolution is a variable of the scene photographed as well as of the camera and film, and the resolution decreases as the contrast decreases. The "theoretical" resolution is important, but it is only a rough measure of the true resolution, which can even vary from one place to another in the same photograph.

With these caveats in mind, the features of interest that were first noted in LO4-191 are compared to their appearance in AS17-288.

The "Ramps"

The Lunar Orbiter frame shows two "ramps" leading down to the floor of the crater from its rim. One is below the letter C (on the north side of the crater) and the other is to the above of the letter E. Looking at the same positions labeled with the same two letters, the ramps are barely visible. The appearance of ramps in the Lunar Orbiter frame has been somewhat exaggerated by the obliqueness of the Lunar Orbiter camera's line of sight to the crater. The "ramps" are foreshortened in their width direction, making them appear narrower than they really are. The lower sun angle in the orbiter frame also accentuates their appearance. In the Apollo photograph of Figure 1B, the two ramps (at the same position relative to the letters C and E) are barely visible. The Apollo photograph has a nearly orthogonal line of sight (i.e., looking almost straight down at the scene), and the widths of the "ramps" are not foreshortened.

The mound at the top of the norther ramp is also much less visible and the unusual serrated appearance in the Lunar Orbiter photograph appears to be the result of some irregular dimpling of the floor of Mare Crisium near the mound. My conclusion here is that these "ramps" are probably just places on the crater wall where the slope of the wall down to the crater floor happens to be shallower than average. However, the odd texture of the southern "ramp" in the orbiter photograph is still interesting.

The "Diamond"

This feature is not brightly lit in the Apollo photograph due to the sun angle, but as can be seen in Figure 1B, there is a squarish dark patch to the left of the letter "B" corresponding in shape and orientation to the "diamond" in the Lunar Orbiter photo. Adjoining this dark patch, the Apollo photograph shows something not revealed in the Lunar Orbiter photograph: the hill adjoining the "diamond" to the east appears conical, with a very circular base on its eastern side, but with a squared-off appearance on the west, where the hill meets the "diamond" in LO4-191H3. More will be said (and shown) about this hill subsequently.

The dark square patch in the Apollo photograph corresponds in size, shape, and position closely to that of the "diamond" in the Lunar Orbiter photograph. Not only is the reality of this feature first noticed in the Lunar Orbiter photo supported by the Apollo photo, but more can be said about it; it is a structure rising to meet the base of the adjacent conical hill, and its slope is somewhat less than the sun's elevation of 26 degrees in the Apollo photograph. If its slope were steeper than this, it would be in complete shadow. If the slope were much less, it would not appear any darker than the surrounding floor of the crater, which presumably is horizontal and would not be visible. The "diamond" is the dark feature outlined in red in Figure 2A, which is a smaller version of the image shown in Figure 2B. The northern (left) edge of this square patch is more sharply defined than the western (botton) boundary.

Figure 2. Click here to download A17288B.jpg  and here for the support data.

Figure 2A  Figure 2B

Furrows and Ridges

In the Lunar Orbiter image of Figure 1A, a number of straight furrows run parallel to each other from just beneath the letter A down toward the center of the crater. In Figure 1B, only a trace of those furrows can be seen at the low magnification of this particular image.

In Figure 2A the ridges are outlined in green. In Figure 2B, which shows the interior of the crater from AS17-288 at twice the magnification of Figure 2A, a number of ridges can be seen of different sizes running down from beneath the position labeled A toward the position labeled X near the central hill. To the lower left of the letter A in Figure 2, a ridge shaped like an upside-down "U" can be seen. The right leg of the "U" is parallel, to and in the same location as the furrows in the Lunar Orbiter photograph. To the left of the letter X in Figure 2 are what appear to be very thin, almost filament-like, features running in the same direction. To the immediate right of the X in Figure 2 two more ridges can be seen running in the same direction, one of them intersecting the conical hill.

To the left of the letter D in Figure 1B, there is another unusually straight ridge (this position is in total shadow in the Lunar Orbiter image). This ridge runs at an angle to the ranks of furrows on the eastern side of the crater. This region is shown at twice the magnification in Figure 3 below.

Figure 3

Click here to download A17288C.jpg and here for support data.

The comparison between the initial Lunar Orbiter frame and the higher-resolution Apollo frame demonstrates the pitfalls of interpretations of features based on a single photograph; the very odd looking ramps in LO4-191H3 seem not so strange in AS17-288. However, the comparison also demonstrates that some of the unusual features in the orbiter photo do persist in the Apollo photo. The "diamond" in LO4-191H3 persists in AS17-288. The appearance of furrows in the Lunar Orbiter photograph may be somewhat supported by the similar orientation of ridges in the Apollo photograph.

AS17-288 is only one photograph in a series of six (286 through 291) showing Peirce. A curious thing about this series is that many features in 288 disappear in the other photographs, including even some of the more substantial-looking ridges, while some very fine-structured striations appear in various places in the other photographs that are not evident in 288. This seems strange since the sun angle is almost the same in all five photographs; only the camera's line of sight differs, and all show nearly orthogonal views of the crater.

These variations also occur in a second series of higher-resolution Apollo 17 photographs of Peirce. Because of the higher resolution (7.7 meters vs. 14.5), these ephemeral features are more easily seen, so images from those photographs, concentrating on the region the "diamond" and the conical hill, are discussed here.

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